ANDROID: cpufreq: times: optimize proc files

The majority of the time spent reading /proc/uid_time_in_state is due
to seq_printf calls. Use the faster seq_put_* variations instead.

Also skip empty hash buckets in uid_seq_next for a further performance
improvement.

Bug: 111216804
Bug: 127641090
Test: Read /proc/uid_time_in_state and confirm output is sane
Test: Compare read times to confirm performance improvement
Change-Id: If8783b498ed73d2ddb186a49438af41ac5ab9957
Signed-off-by: Connor O'Brien <connoro@google.com>